Compact light sensors are known that measure a variety of properties of light beams. For example, a photodiode can measure the intensity of an incident light beam by converting incident light into a signal that has a voltage or current that depends on the intensity of incident light. Photodiodes and similar sensors can also be used with optical filters or separators to separate and measure polarization and spectral components of a light beam. Guided-mode resonance (GMR) filters, for example, have been used in sensors that detect light of a particular frequency. One configuration for a compact GMR filter includes a waveguide layer and a grating layer, which can be fabricated on an integrated circuit chip. The grating layer typically reflects part of an incident light beam, transmits part of the incident light beam, and diffracts part of the incident light beam. The diffracted part enters the waveguide layer, but through further interaction with the grating layer, the light in the waveguide can diffract out and interfere with the transmitted part and/or the reflected part. A GMR filter is generally designed to have a “resonance” such that incident light having a resonant frequency is coupled into the waveguide structure with high efficiency, causing the diffracted part of the light to destructively interfere with the reflected part and/or the transmitted part, changing the overall transmission/reflection properties of the structure dramatically around the resonant frequency. For instance a grating layer which, prior to being patterned, would mostly transmit light, becomes highly reflective around the resonant frequency when the holes are indeed present. However, incident light at a non-resonant frequency is not significantly coupled into the waveguide and continues to be transmitted through the grating. Accordingly, resonance gratings can be used as optical filters.
Compact light sensors are also known that measure relative characteristics of a beam. For example, an array of photodiodes or other light sensors can measure the position of a light beam relative to the sensor array simply by determining which sensor or sensors in the array detect light from the beam. Such measurements are useful in alignment systems. In particular, if the sensors detecting a light beam are away from a target location on the sensor array, an alignment system can provide a warning regarding the alignment of the light beam or can actively shift the light source or the sensor array to improve alignment. Incidence angle sensors can also be useful in alignment systems, but a typical solution for measuring the incidence angle of a light beam requires optical elements such as cube corner reflectors that are not easily miniaturized to the sizes of integrated circuits.
Use of the same reference symbols in different figures indicates similar or identical items.